How to read the material library
CHIP describes how each material behaves: whether it burns, tears out, melts or lifts, and which bit and strategy suit it. It does not publish a feed and speed table, because a number that ignores your machine, your bit and your workholding is worse than no number at all.
What each material page tells you
- How it machines. Hard or soft, abrasive or kind, stable or moving.
- What goes wrong with it. Burning, tearout, melting, chipping, lifting, fuzz.
- Which bits suit it and which are a bad idea.
- Which direction to push the settings relative to a general wood baseline.
- Workholding and finishing notes specific to that material.
What it does not tell you
A single feed rate and RPM per material. Here is why that number would be useless:
Two people both cutting hard maple with a 1/4 inch two flute upcut. One has a 200 pound steel frame machine with a 2.2 kW spindle. The other has an aluminium extrusion gantry with a trim router. The correct feed rate differs by a factor of three or more, and both of them are right. A published number would be wrong for at least one of them, and probably both.
Finding your own numbers, in twenty minutes
This is worth doing once per material you use often. It gives you numbers that are true for your machine, and they stay true.
- Start from a chipload, not a feed rate. Take your bit maker's published range for the material and diameter, and pick the middle of it.
- Pick an RPM your machine runs comfortably. Middle of the dial for most routers.
- Calculate the feed. Use the chipload calculator. If the answer is beyond your machine, drop the RPM or use a single flute bit rather than accepting a low chipload.
- Cut a test. A few short slots at a depth per pass of about half the bit diameter, in scrap of the same material.
- Read it. Chips not dust. Bit warm not hot. Edge clean not brown. Sound steady not screaming.
- Push one variable. Increase depth per pass by 25 percent and repeat until something complains. Back off one step.
- Write it down. Material, bit, flutes, RPM, feed, depth. Keep the card by the machine.
After three or four materials you stop needing the procedure, because you will have learned what your machine sounds like when it is happy.
The variables that move the answer most
| Variable | Effect | Direction |
|---|---|---|
| Machine rigidity | Very large | Less rigid means lighter depth and stepover, not slower feed |
| Bit diameter | Very large | Smaller diameter means smaller chipload and much less depth |
| Flute count | Large | More flutes needs proportionally more feed |
| Bit sharpness | Large, and invisible | A dull bit needs more force and makes far more heat |
| Depth of cut | Large | The first thing to reduce when anything is wrong |
| Species and density | Moderate | Harder means lighter cuts and more attention to heat |
| Moisture content | Moderate and underrated | Wet wood gums up, moves and cuts badly |
| Grain direction | Moderate, and decides tearout | See grain direction |